What is filtration?

Filtration it is the procedure by which separates a solid in suspension in a liquid. It is used to remove solid impurities from a solution or to isolate an organic solid. In the laboratory of Organic Chemistry, there are two main types of filtration, gravity filtration and vacuum filtration or suction.

Both filtration methods present advantages and disadvantages. But are used depending on if we are interested in the sample is the filtering, or the solid. Filtering refers to the liquid that has passed through a filter paper.

Gravity filtration

It is used when we want to preserve, a liquid that we want to separate a solid (solid impurities from an organic liquid). The impurity can be a drying agent (Na2SO4), or an undesired by-product or a reactive solid is to be disposed of. The gravity filtration can be used to obtain solid products, although it is generally employed in the vacuum filtration for this purpose because it is faster. The procedure that is used is the so-called “gravity filtration in hot” to separate the impurities insoluble in a solution of hot.

Hot filtrations require a flutted filter paper and a careful attention to the procedure to keep the solution hot, but with care so that the solvent does not evaporate. If we use non-water solvents (organic solvents), you must attach a refrigerant of reflux which prevents from evaporation, and also whenever the solvent is flammabled to avoid accidents (see how to mount a reflux).

The equipment needed to perform this filtration is:

  • Conical funnel.
  • Ring clamps to attach the funnel.
  • Erlenmeyer flask (or other container) to collect the liquid after filtration.
  • Fluted filter paper

gravity filtration pleated filter paper conical funnel conical beaker
The solid (disposable) is retained on the filter paper set in the conical funnel, through which the solution is recovered.

Video about Gravity Filtration

Preparation of the filter paper

The fluted filter paper is made from a square piece of filter paper by cutting it to form a circle. Then, the paper is folded as shown in the Figure.

how to prepare pleated filter paper step by step

The purpose of the folds is to facilitate the passage of the liquid through the paper to increase the effective area of filtering. The size of the filter should not protrude above the rim of the conical funnel.

Filtration process

  • The iron ring is fixed to the rack and placed the funnel conical.
  • The height of the iron ring is regulated so that the output of the funnel into the collection container (Erlenmeyer flask or beaker).
  • Once prepared, the filter pleats are placed in the funnel, conical.
  • The liquid is poured into the filter without allowing it to overflow. This can be done using a bar of police (see Figure).
  • Let the liquid falls by gravity on the receptacle.
  • Finally, you add a small amount of solvent to wash the solid remaining
    in the filter.

Vacuum filtration

Unlike the gravity filtration, vacuum filtration is used to separate a solid suspended in a solvent when what you want in the mix is solid. For example, obtaining crystals in a procedure recrystallization. The vacuum filtration is faster than gravity filtration, because the dissolution and the air are forced through the filter paper by the application of vacuum. You should not use the vacuum filtration to filter a solid from a liquid is the liquid that you want, and if the liquid has a temperature close to its boiling point. Any solvent with low boiling points will evaporate with the effect of the vacuum in the kitasato.

The equipment needed to perform a filtration vacuum is the following:

  • Büchner: is a funnel-shaped sieve cylindrical (of porcelain or plastic) that has small holes in a flat bottom (see Figure). Place a piece of filter paper round in the Büchner.
  • Kitasato flask: resembling a Beaker in its tapered shape, but in the upper part protrudes a tube finished off to connect it to the outlet of a vacuum.
  • Perforated stopper: it is placed between the Büchner and the Kitasato flask to fit well when it is performed in the vacuum.
  • Vacuum trap: this device is inserted between the vacuum source and the Kitasato to prevent the liquid that remains in the Kitasato go to the machine that generates the vacuum.
  • Fluted filter paper round.
  • Vacuum pickup.

The filter flask must be secured to a rack or metal bracket by means of a clamp with nut.

kitasato vacuum filtration büchner rubber cone

Video about Vacuum Filtration

Filtration process

  • Cut a piece of filter paper round, making sure that the filter is the correct size and it does not touch the walls of the Büchner or leaving the holes unfilled.
  • Once placed the filter paper, the Büchner adjust to the Kitasato flask using the perforated stopper, setting the Kitasato flask with a clamp and nut to the grid.
  • Connects the Kitasato flask the trap of making empty.
  • The filter paper is moistened with a few drops of the same mixture of solvents that are filtered out so that it is completely stuck to the bottom of the Büchner.
  • The mixture to be filtered is poured into the Büchner, and as in the gravity filtration, the solid is dropped with the help of a glass rod.
  • Pour all of the liquid, using more solvent if it is necessary to drag the solid remaining that are attached to the vessel containing the mixture.
  • Once the filtration is complete, disconnect the Kitasato flask from vacuum line, and then close vacuum gauge.

Vacuum filtration can also be performed for solid disposable. For example, when the gravity filtration is slow.

When the solids to the filter have a particle size of very small, the filter paper may not have a pore size small enough. In these cases, use funnels, cylindrical, with a plate or filter funnels, conical, called Hirsch funnels.


La gravity filtration it is typically used when we are interested in the filtering, while the vacuum filtration is used when we are interested in the solid.

  • Gravity filtration is preferred when applying vacuum could carry small solid particles through the pores of the filter paper, producing potentially a filtering contaminated with the solid compound.
  • Vacuum filtration is preferred when the gravity filtration is much less effective to remove the residual liquid of the solid on the filter paper.